How do hoseless units transmit data?

[mopyfl]

I have made my own computers and dataloggers, including a favorite I have used for about 3 yrs (on the original battery.) I would like to update my design to a hoseless type. While I have most of the hardware needed for this, I am still at a loss as to the best transmission technology to use between the tank sensor and the computer.
What are the chances that someone out there either knows how this is done, or has taken a tank mount transducer apart to see? I can think of several methods to implement, being RF, acoustics and IR, but I would really like to know how it is done in the real world before trying to re-invent the wheel.

Yes, I do use standard guages as back-up, so not to worry, I am a qualified engineer and will not kill myself, so if anyone has info on this, please share!

Thanks,
Chris

 

[jplacson]

Well most, if not all, hoseless AI computers use RF technology. What frequency is used, or RF strength isn't standard and each manufacturer uses their own design and what they feel is best.

The main problem with RF is that it hates water... all the more salt water. Most RF transducers are built for in-air use so they operate at higher frequencies. For the best underwater transmission, ULF should be used...same as military subs, and the like. But I don't know if there are any commercial ULF units for sale, and if they are power efficient enough to be used on a regular basis (most commercial hoseless AI transmitters can last from 2-5 years before a battery change)

If you do design a better transmitter... one that sips power... and can transmit reliably in salt water up to 6 feet away... and isn't affected by underwater strobes... you might have some luck selling your design to any of the hoseless computer manufacturers! Good luck! And I hope to see your design in a future hoseless AI computer!

 

[mopyfl]

Thanks for replying.
Yeah, RF underwater is a real bummer, salt water especially, since it is a conductor and transmitting through it is like pumping output into a short circuit. This is why I believe the power gets gulped - a lot of power is needed to get a strong enough signal through short distances in this environment. A Cal State Prof. told me not to even try w/ RF. (Humbug! - what does he know!)

I have been thinking of trying an acoustic set-up which is basically a take off on the old acoustic modems used when you had to clip transducers to telephone handsets to get computers to talk to each other in the old days. The problem here is that the low frequency needed may be audible to divers and sea life. Hmmm.....Perhaps I will go back to RF.

I recently bought a toy submarine from radio shack that worked with the transmitter out of the water to sub depths of about 3 or 4 feet. The whole transmitter and receiver pair couldn't have cost more than $1.00 to build...I didn't want to believe that expensive dive computers could be made with such lo-tech communications!

The best way to save power is to transmit in short bursts, then put the unit to sleep until it's time to work again. I have already accomplished this with my current computer, but I understand that transmit bursts will use more power yet. I think it's all about slick programming skills. We'll see if I'm up to the task. DO you know if any of the manufacturers publish the output power oin milliWatts?....perhaps in owner's manuals?

C

 

[jplacson]

Nope, AFAIK no manufacturer has ever published details on their transmitter units. They do transmit in bursts (usually once every second) And most of them may have problems transmitting from the tank to your wrist... a distance of about 2-3 feet. Although 70% of the time, they seem to work fine, sometimes they just stop working for a while then resume again.

Acoustic coupling may work.. that's kinda like going on the lower frequencies... the lower the better... since audible frequencies travel farther in water. (Maybe like an audible sonar ping?) Also, slowing the refresh rate to maybe once every 5 seconds could save battery life.